1. Field of the Invention
This invention relates to a process for isolating a protein fraction, to the protein fraction isolated by this process and to the use of the said fraction.
2. Description of the Prior Art
The separation of substances of high molecular weight, such as proteins for example, from substances of low molecular weight, such as salts, sugars, etc., can be carried out satisfactorily in a number of ways, in particular by microbial separation, by chemical separation and by physical separation. The formation of curd in cheese making is the classic example of microbial separation. It involves inter alia a transformation of casein, and leads to the special products which cheeses are. Chemical separation generally necessitates the intervention of third substances which, as a rule, have to be subsequently eliminated and which, in many cases, can only be elimininated with considerable difficulty.
These methods frequently amount to self-developing operations in the sense that, once separation has been initiated, matters follow their course autonomously, and culminate in a specific end product without any opportunity for effective intervention.
Unlike microbial and chemical separation, which often involve a change of phase and/or a change of state, physical separation generally takes place without effecting the proteins in any way. There are at present two forms of physical separation, namely separation on a molecular sieve or separation by ultrafiltration. Except for the adsorption phenomena, molecular-sieve separation, in which a solution containing the proteins is passed through a column filled with a suitable gel, is an operation of the all-or-nothing type, because any substance with molecular dimensions larger than the pores of the sieve is not retained, whilst smaller substances are trapped and blocked. Separation by ultrafiltration is not an instantaneous. Although the substances are distributed progressively on either side of a membrane in dependence upon their molecular dimensions, only substances of small molecular dimensions being able to pass through the membrane, the segregation produced results from a difference in tendency. If in practice it is desired to obtain a product of high concentration, it is necessary to carry out numerous individual separations on as many series-arranged membranes, or to work in a closed loop by recycling. However, polarisation phenomena tend to occur at the membranes, rendering them ineffective. These phenomena are more pronounced, the higher the concentration of the solution being treated. Finally, the viscosity of the product increases with its solids content. This increase in viscosity can only be counteracted with some difficulty by an increase in temperature, a factor to which proteins are extremely sensitive. For these various reasons, it is normally not possible, for example, to prepare by the ultrafiltration of milk an end product whose solids content exceeds approximately 25%, i.e. a protein : lactose ratio of around 3 : 1.
An object of the present invention is to obviate the disadvantages and limitations referred to above.